Resonance in the Peptide Bond

Introduction to Protein Structure

Resonance of the Peptide Unit

The amide structure has two resonance contributors:

Because the bond between the carbonyl carbon and the nitrogen has a partial double bond character, rotation around this bond is restricted. Thus, the peptide unit is a planar, rigid structure and rotation in the peptide backbone is restricted to the bonds involving the a carbon:

The rigid, planar nature of the peptide unit has implications for the detailed three-dimensional strcutrue of peptides, as we will see later.